1. Field of the Invention
The present invention relates to a soft-start circuit and, more particularly, to a sequential soft-start circuit for multiple circuit channels.
2. Description of the Prior Art
For a wide range of integrated circuit designs, soft-start circuits are usually provided to supply a soft-start signal for limiting an inrush current upon activation and protecting the integrated circuits from being damaged thereby.
FIG. 1(A) is a circuit diagram showing an integrated circuit chip 10A provided with a conventional soft-start circuit 12. The integrated circuit chip 10A has a main circuit 11 and a soft-start circuit 12. The main circuit 11 executes the very function that the chip 10A is designed for. In FIG. 1(A), an output signal OUT is illustrated to symbolize the function. For example, if the integrated circuit chip 10A is designed for supplying a current to a load at a regulated voltage, then the main circuit 11 may be implemented by a variety of voltage regulators such as switching regulators or charge pumps, which are well known in the art. The soft-start circuit 12 applies a soft-start signal VS to the main circuit 11 for limiting the inrush current. Normally, the soft-start signal VS is a gradually increasing voltage signal. As shown in FIG. 1(A), the soft-start circuit 12 has a soft-start current source IS and a soft-start capacitor CS. The soft-start current source IS charges the soft-start capacitor CS such that the potential difference across the soft-start capacitor CS gradually increases and serves as the soft-start signal VS. Because of its size, the soft-start capacitor CS is typically installed on the outside of the integrated circuit chip 10A and connected to the soft-start current source IS through an electrical pin PS of the chip package.
FIG. 1(B) is a circuit diagram showing a multi-channel integrated circuit chip 10B. The multi-channel integrated circuit chip 10B has n main circuit channels 11-1, 11-2, to 11-n, which are independently controllable with respect to each other, wherein the constant n is an integer larger than or equal to 2. In order to protect the circuit from being damaged by the inrush current upon activation, each of the main circuit channels 11-1, 11-2, to 11-n is correspondingly provided with one of soft-start circuits 12-1, 12-2, to 12-n for generating one of soft-start signals VS1, VS2, to VSn. Since in each of the soft-start circuits 12-1, 12-2, to 12-n one of soft-start capacitors CS1, CS2, to CSn is provided to be correspondingly charged by one of soft-start current sources IS1, IS2, to ISn for generating one of the soft-start signals VS1, VS2, to VSn, the multi-channel integrated circuit chip 10B must have a corresponding number of electrical pins PS1, PS2, to PSn for providing the necessary connections with the external soft-start capacitors CS1, CS2, to CSn. As a disadvantageous result, the chip package has to be made larger for accommodating such many electrical pins PS1, PS2, to PSn. In addition, the total cost is raised due to the large number of the soft-start capacitors CS1, CS2, to CSn.